Ultra-endurance off-road events are increasing in popularity and availability. Here is a list of things to test and perfect in your training in order to successfully step up to the distance.
10-plus hours in the saddle off-road will chafe even the most hardened rear ends. Apply a lubricant before you start the day. My favorite is Udder Butter. Dual suspension bikes or suspension seat posts are nice.
You will be riding through the heat of the day losing electrolytes through your sweat. Replacement is essential. I like Endurolyte capsules by Hammer Nutrition. Take two with breakfast and one per hour while riding.
Keep your engine fueled consistently in order to chug along for hours. A good starting point is 0.5 grams of carbohydrate per pound of body weight per hour. You may ride better on more or less than this.
Weigh yourself before and after long rides to calculate if you drank enough. Drink 24 ounces for every pound lost.
Your core supports you on the bike for many hours and needs to be super strong to avoid the dreaded lower-back ache. Focus on mobility and stability with functional core-training exercises such as pikes on an exercise ball or one-leg push-ups.
Specifically strengthen your psoas muscles, which run from the front of your upper leg, through the pelvis to the lower spine. (The psoas, together with the iliacus muscles, are known as the hip flexors.) There are a variety of stretches and exercises (such as lunges) and yoga positions (such as the "Warrior" position), that target the hip flexors.
Do long rides up to 10 hours in duration to test all of the above and, most importantly, to figure out pacing. You need to ride conservatively for the first 50 miles to avoid blowing up at mile 70.